Monosaccharides are staples in commerce with a diversity of uses, for example, the aldohexose glucose is used as a sweetener, as are the ketohexoses fructose and sorbose. Other monosaccharides, such as the pentoses arabinose and xylose, as well as glucose, often are used as the major component of fermentation media. Still other monosaccharides, such as mannose, are reduced to polyols such as mannitol and sorbitol, used as humectants and excipients in tablets.
The monosaccharides themselves generally are obtained by hydrolysis of polysaccharides, i.e., polymers having monosaccharides as their repeating units. Hydrolysis of polysaccharides sometimes is chemical, sometimes is enzymatic, and sometimes is a combination of both. For example, hydrolysis of starch, or saccharification as the process more often is called, may be a combination of acid and enzymatic (alpha-amylase) hydrolysis to afford partially hydrolyzed reaction mixture containing some mono-, di-, and trisaccharides, but composed mainly of polysaccharides with 4 or more monomeric units. Polysaccharides composed of n monomeric units often are referred to as having a "degree of polymerization n" or with the notation DPn; in this context partially hydrolyzed starch is mainly DP4, DP5, etc., collectively here designated as DP4+. The partially hydrolyzed starch, or a thinned starch as it is often called, is then further hydrolyzed enzymatically by amyloglucosidase (AG, or glucoamylase) to afford a mixture rich in monosaccharides (DP1), but also containing disaccharides (DP2), trisaccharides (DP3), and higher polysaccharides (DP4+).
To obtain pure monosaccharides, or nearly so, it is necessary to separate them from the di-, tri-, and higher polysaccharides in an aqueous solution which often contains the hydrolytic enzymes. Among the methods of separation which have been utilized is included membrane separation, that is, ultrafiltration using a membrane with a molecular weight cutoff low enough to reject materials of molecular weights substantially higher than the monosaccharide, but high enough to let the monosaccharide pass through at a high flux. The delicate balance between selectivity in rejecting unwanted components and a high flux through the membrane of a permeate rich in monosaccharides has spurred continued development of membranes having the requisite characteristics. During the course of the program to evaluate such membranes we encountered the phenomenon of concentration polarization whereby membrane rejected material, such as DP4+, tends to have locally extraordinarily high concentration at or near the membrane surface. Stated differently, there is often a steep, exponential gradient in the concentration of rejected species from the bulk of the feedstock toward the membrane surface, so that the concentration of such species at the membrane is many-fold greater than their concentration in the solution used as the feed. Generally such concentration polarization is undesirable since it may cause precipitation of the species congregating at the membrane surface leading to pore plugging, reduced flux, and a varying rejection coefficient. In fact, such was our observation when the feedstock was a synthetic mixture of DP1, DP2, DP3, and DP4+ prepared to match the characteristics of various partially hydrolyzed starch solutions. Concentration polarization can be minimized, for example, by transforming the feed flow from laminar to turbulent flow, or by reducing the height of the channel through which feedstock flowed. The reduction of concentration polarization was our initial task, and in fact reduction led to improved separation of monosaccharides through the membranes.
To our astonishment, when a feedstock of partially hydrolyzed starch containing the enzyme amyloglucosidase was passed over an ultrafiltration membrane under conditions where concentration polarization could be expected it was found that the separation of monosaccharides was substantially improved relative to the case where enzyme was absent. This unprecedented observation was examined in great detail, leading to our discovery that within a given, well defined range of AG enrichment at or near the membrane surface arising from concentration polarization, the separation of monosaccharides from the aforementioned mixture is enormously enhanced with little loss in flux. This stunning discovery has led to an improvement in the method of separating monosaccharides by ultrafiltration. Our discovery has further led to an improvement in the process of making glucose by partial saccharification of starch, separating the effluent stream into one enriched in monosaccharide and a stream depleted in monosaccharide but enriched in higher saccharides, and recycling the latter to the saccharification reactor. Such an underlying process recently has been described in U.S. Pat. No. 4,594,322 (compare U.S. Pat. No. 4,511,654) and will be further elaborated upon within.